The present invention relates to a two-way data communication system in which data communication is carried out between a central facility, hereinafter referred to as "a center", and a plurality of terminal units. More particularly, the invention relates to a line monitoring device in a CATV system, in which two-way data communication is carried out, which monitors the levels of data signals which are transmitted from the terminal units to the center, hereinafter referred to as "up-data signals" or "up-signals", when applicable.
In general, in a CATV system, the center is connected through cables to the terminal units provided in subscribers' homes so that television signals received by the center or television signals of programs produced by the center are transmitted through the cables to the television sets in the subscribers' homes. This system is considerably advantageous in that, since the center is connected through lines or cables to the terminals units as described above, data can be transmitted between the center and the terminal units, and not only are television signals transmitted to the television sets in the subscribers' homes, but also the system has a wide range of applications.
In such a CATV system which is capable of carrying out two-way data communications, the center is connected through lines or cables to the various terminal units so that the center can receive data signals from the terminal units and vice versa. In order to carry out two-way data communication satisfactorily, it is essential that the terminal units operate satisfactorily. Accordingly, it is necessary to monitor the levels of data signals on the lines at all times. Heretofore, the levels of data signals on the lines were monitored according to a method in which pilot signals are applied to the lines of the CATV system and the levels of the received pilot signals are monitored.
However, that method is not economical because the provision of a special device is necessary to apply pilot signals to the lines and a separate frequency or frequencies as well as time are required for monitoring the levels of data signals on the lines.